1. Field of the Invention
This invention pertains generally to radio frequency (RF) transmission over optical carriers and more specifically to the generation of single sideband signals with suppressed carrier over a wide RF bandwidth for transmission over optical fibers.
2. Description of the Related Art
In electronic communications, single sideband (SSB) transmission is a well-known technique for reducing bandwidth and power requirements as well as "fading" due to dispersive transmission environments. When combined with carrier suppression (SC), this technique can enhance link gain in an amplified system. These benefits can be similarly realized in photonic links where RF signals are multiplexed onto an optical carrier and transmitted via a fiber optic cable, which has advantages over coaxial cable in terms of bandwidth, incremental loss, and electromagnetic interference (EMI) immunity. For micro- and millimeter-wave modulation frequencies, SSB becomes a useful way of overcoming fiber chromatic-dispersion effects (Fading in the fiber) while also suggesting new applications outside communications. (SEE, Smith et al., OVERCOMING CHROMATIC-DISPERSION EFFECTS IN FIBER-WIRELESS SYSTEMS INCORPORATING EXTERNAL MODULATORS, IEEE Trans. Microwave Theory Tech., Vol. 45, pp. 1410-1415, 1997.)
From communications theory, it is well known that a small-signal phase- or amplitude-modulated (PM or AM) carrier will appear on a spectrum analyzer as shown in FIG. 1a. This double sideband (DSB) spectrum can be easily generated using a laser and an electro-optic phase or amplitude modulator (two phase modulators combined in a Mach-Zehnder configuration) driven by an RF generator. In a Mach-Zehnder modulator, depending upon modulator bias and the relative phase of the RF inputs, the phase relationship between the three peaks can be a combination of those relationships illustrated by the phasor diagram FIG. 1b. With the carrier as a reference phasor, the sidebands will rotate in opposite directions: if they are phased such that the total sideband phasor is always parallel or antiparallel to the carrier, then the result will be AM; if they are phased such that the total sideband phasor is always perpendicular to the carrier, then the result will be mostly PM with a small amount of AM. True PM is comprised of additional harmonic sidebands such that the resultant (carrier plus sidebands) actually swings in pendulum fashion with constant amplitude.
Chromatic dispersion is a well-known limitation on the transmission distance achievable in photonic (optical carrier-based) links. While recognized for some time as a problem for digital systems, recent interest has been driven by the wireless/mm-wave market. (SEE, Yonenaga et al., A FIBER CHROMATIC DISPERSION COMPENSATION TECHNIQUE WITH AN OPTICAL SSB TRANSMISSION IN OPTICAL HOMODYNE DETECTION SYSTEMS, IEEE Photon. Technol. Lett., Vol. 5, pp. 949-951, 1993; Smith et al., opcit., Hofstetter et al., DISPERSION EFFECTS IN OPTICAL MILLIMETER-WAVE SYSTEMS USING SELF-HETERODYNE METHOD FOR TRANSPORT AND GENERATION, IEEE Trans. Microwave Theory Tech., Vol. 43, pp. 2262-2269, 1995; Gliese et al.,CHROMATIC DISPERSION IN FIBER-OPTIC MICROWAVE AND MILLIMETER-WAVE LINKS, IEEE Trans. Microwave Theory Tech., Vol. 44, pp. 1716-1724, 1996; and Park et al., ELIMINATION OF THE FIBRE CHROMATIC DISPERSION PENALTY ON 1550 NM MILLIMETER-WAVE OPTICAL TRANSMISSION, Electron. Lett. Vol. 33, pp. 512-513.) Dispersion D (in ps/km-nm) results in a differential phase delay .phi.=.pi.D.lambda..sup.2 Lf.sup.2 /c between one spectral component at wavelength .lambda. and another such that the difference increases linearly with distance L and quadratically with the frequency separation f. (See, Gliese, opcit.) Referring again to FIG. 1b, this means that the oscillation direction of the total sideband phasor rotates linearly with L, periodically transforming AM to PM and back. For an intensity-modulated direct-detection (IM-DD) link, this results in a sinusoidal variation of link gain with L. The frequency response at a particular length will also be sinusoidal with quadratically increasing "phase". (SEE, Smith and Park, opcit.)
If a SSB scheme is used to eliminate one of the sidebands, the other sideband will rotate alone, producing a combination of AM and PM of lesser amplitude. The phase-insensitive DD receiver will then recover the AM without the periodicities associated with carrier-to-sideband dispersion. It is noted that this is not generally a complete solution to the dispersion problem. If the single-sideband is sub-carrier modulated (SCM) or the optical carrier is baseband modulated with a large enough information bandwidth, the information itself will be "dephased" over the fiber length. (SEE, Hofstetter, and Yonenaga, opcit.) Over a large distance, the optical linewidth shared by the carrier and the sideband is also decorrelated resulting in increased phase noise. (SEE, Gliese, opcit.) There are a variety of optical techniques (e.g., chirped gratings, dispersion-compensating fiber) which can be applied to reduce this effect.
Some techniques for photonic SSB generation involve elimination of one sideband from a DSB output. These include using a fiber Bragg grating (FBG) notch filter or, in the case of baseband modulation, using a Mach-Zehnder-type filter with a ring resonator on one arm to obtain a sharp cutoff band-reject filter. (SEE, Park and Yonenaga. opcit.) Another technique treats the DSB-SC output of a null-biased Mach-Zehnder modulator (MZM) as a SSB output with twice the modulation frequency and utilizes a delay-line filter to separate and modulate only one sideband. (SEE, Hofstetter, opcit.) The difficulty with these techniques is that they require filter designs which are matched to the frequency separations of interest and are therefore limited in bandwidth.
An alternative approach is to generate SSB output via cancellation within a dual-drive MZM. As shown in FIGS. 2a and 2b where the carrier phasors correspond to each arm of the device, if the modulator is biased at quadrature and the RF drives are .pi./2 out of phase, then either the upper or lower sidebands will cancel. This technique has been applied successfully to 80 km of fiber without dispersion-induced power degradation. (SEE, Smith, opcit.) This technique can be used with a traveling-wave modulator placed at the center of a polarization-maintaining (PolM) fiber loop joined by a PolM fiber coupler (a Sagnac interferometer) in order to supress the carrier as well. (SEE, Frankel et al., OPTICAL SINGLE-SIDEBAND SUPRESSED-CARRIER MODULATOR FOR WIDE-BAND SIGNAL PROCESSING; IEEE J. Lightwave Technol., Vol. 16, pp. 859-863, 1998.) The traveling-wave device generates sidebands more efficiently in the direction for which the RF signal and optical carrier are copropagating, thereby avoiding loop cancellation of the sidebands. Note that SSB with carrier can be recovered simultaneously (using a circulator) from the reflection port of the PolM fiber optic coupler. A problem with the single-MZM Sagnac SSB-SC configuration is that, because the sidebands propagate in the forward direction only, true carrier cancellation can only be achieved at a single RF power level, limiting the range of usable drive levels.
Integrated optical devices utilize materials which have been processed to create light-guiding regions with cross-sectional dimensions approximating the light wavelength as structures for generation, manipulation, and detection of light wave (SEE, Hutcheson et al., INTEGRATED OPTICAL CIRCUITS AND COMPONENTS, Dekker; New York; 1987.) As such a material, LiNbO3 is a common substrate because of its excellent electro-optical and waveguiding properties. In commercial optical modulators, this material is inlaid with waveguides typically using either an impurity in-diffusion or ion exchange method. Depending upon the surface orientation of the crystal substrate, electrodes are laid out directly above or above and beside the waveguide to influence the phase of the lightwaves propagating therein via the electro-optic effect. Such a device, with a single waveguide acts as an electrically-driven optical phase modulator and can be butt-coupled to fiber pigtails for integration into fiber optic systems. In other devices, the waveguides in these modulators are laid out in a splitter-parallel-combiner sequence to form a Mach-Zehnder interferometer, wherein the applied field influences the relative phase between the lightwaves propagating in the parallel arms. Thus, an applied electrical RF signal can induce an optical intensity fluctuation at the same frequency.
In order to improve the coupling efficiency of the electric field-optical field interaction at higher speeds, traveling-wave electrodes (microwave transmission lines parallel to the optical waveguides) are used instead of bulk (area, capacitor-like) electrodes. Designs which improve the propagation velocity match between the electrical and optical waves also enhance coupling efficiency and thereby improve modulation bandwidth. Because of the velocity match property, light in such devices is modulated more efficiently in one propagation direction than the other. For reference, this is called the preferred direction of the modulator.
Intensity (amplitude) modulators can be fashioned with a single RF drive or dual RF drives. In the former device, the electrodes are typically designed such that an antisymmetric field is generated across both parallel arms to produce a "push-pull" relative phase modulation which preserves the average relative phase rather than using an independent set of electrodes to drive each arm as in the latter device. However, the same effect can be generated in the dual-drive MZM by driving its RF inputs with the same signal 180 degrees out of phase. This "push-pull" modulation is preferred because phase disturbance is undersireable in an amplitude modulator.